The invention relates to a metallocene catalyst useful in preparing syndiotactic/atactic block polyolefins and to the polymers so obtained. The catalyst comprises a bridged monocyclopentadienyl metallocene in which the cyclopentadienyl ring is substituted symmetrically with respect to the active coordination positions. The invention further includes a process of preparing syndiotactic/atactic block polyolefins that comprises the use of the disclosed catalyst.
The present invention also provides for a process for polymerizing olefins having three or more carbon atoms to produce polymers with syndiotactic/atactic stereochemical configurations. The catalyst and process of the invention are particularly useful in polymerizing propylene to form a syndiotactic/atactic block polypropylene.
As known in the art, syndiotactic polymers have a unique stereochemical structure in which monomeric units having enantiomorphic configuration of the asymmetrical carbon atoms follow each other alternately and regularly in the macromolecular main chain. Syndiotactic polypropylene was first disclosed by Natta et al. in U.S. Pat. No. 3,258,455. The Natta group obtained syndiotactic polypropylene by using a catalyst prepared from titanium trichloride and diethyl aluminum monochloride. A later patent to Natta et al., U.S. Pat. No. 3,305,538, discloses the use of vanadium triacetylacetonate or halogenated vanadium compounds in combination with organic aluminum compounds for producing syndiotactic polypropylene. U.S. Pat. No. 3,364,190 to Emrick discloses a catalyst system composed of finely divided titanium or vanadium trichloride, aluminum chloride, a trialkyl aluminum and a phosphorus-containing Lewis base as producing syndiotactic polypropylene. U.S. Pat. No. 4,892,851 disclosed a metallocene catalyst for producing highly crystalline syndiotactic polyolefins.
As disclosed in these patent references and as known in the art, the structure and properties of syndiotactic polypropylene differ significantly from those of isotactic polypropylene. The isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows: 
Another way of describing the structure is through the use of NMR. Bovey""s NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each xe2x80x9cmxe2x80x9d representing a xe2x80x9cmesoxe2x80x9d dyad or successive methyl groups on the same side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Using the Fischer projection formula, the structure of a syndiotactic polymer is designated as: 
In NMR nomenclature, this pentad is described as . . . rrrr . . . in which each xe2x80x9crxe2x80x9d represents a xe2x80x9cracemicxe2x80x9d dyad, i.e., successive methyl groups on alternate sides of the plane.
The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and, like the isotactic polymers, are insoluble in xylene.
This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer that is soluble in xylene. Atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product. While it is possible for a catalyst to produce all three types of polymer, it is desirable for a catalyst to produce predominantly syndiotactic or isotactic polymer with some atactic block fractions.
The invention provides for a new syndiotactic/atactic block homopolyolefins and particularly a new syndiotactic/atactic block homopolypropylene.
The present invention also provides a catalyst and process for preparing syndiotactic/atactic block polyolefins, and more particularly syndiotactic/atactic block polypropylene. The catalyst and process can each be adapted to produce a polymer with differing syndio-/atactic block ratios.
The catalyst comprises a metallocene, i.e., a metal derivative of a cyclopentadiene, and an ionizing agent. The metallocene compound contains only one substituted cyclopentadienyl ring and is of the general formula:
Rxe2x80x3(C4Rxe2x80x2mC5C4Rxe2x80x2n) XMeQ
wherein X is an hetero-atom ligand with one or two lone pair electrons and selected from the elements of Group VA or VIA and is preferably nitrogen, phosphorus, oxygen or sulfur, which can be substituted or non-substituted; (C4Rmxe2x80x2C5C4Rnxe2x80x2) is a fluorenyl or a symmetrically substituted fluorenyl or cyclopentadienyl ring; Rxe2x80x2 is hydrogen or hydrocarbyl radical having from 1-20 carbon atoms, a halogen, an alkoxy, and alkoxy alkyl or an alkylamino or alkylsilylo radical, each Rxe2x80x2 may be the same or different and m and n independently are 0, 1, 2, 3 or 4, with the proviso that the bilateraly symmetry is maintained; Rxe2x80x2 is a structural bridge between the X and (C4Rxe2x80x2mC5C4Rxe2x80x2n) ring to impart stereorigidity and, preferably is a silyl or a hydrocarbyl biradical having at least one silicon or carbon atom to form the bridge; Q is a hydrocarbyl radical, such as an alkyl, aryl, alkenyl, alkylaryl or arylalkyl radical having 1-20 carbon atoms or is a halogen; Me is a Group IIIB, IVB, VB, or VIB metal as positioned in the Periodic Table of Elements; Me can be in any of its theoretically possible oxidation states. Preferred metals for the catalyst of the invention are Zr, Ti, Hf.
The term xe2x80x9csymmetricallyxe2x80x9d shall mean that the local bilateral symmetry of the active polymerization sites is essentially maintained.
The present invention further provides a process for producing syndiotactic/atactic block polyolefins, particularly syndiotactic/atactic polypropylene. The process comprises utilizing at least one of the catalysts described by the above formula and introducing the catalyst into a polymerization reaction zone containing an olefin monomer. In addition, a cocatalyst such as alumoxane may be introduced into the reaction zone. Further, the catalyst may also be pre-polymerized prior to introducing it into the reaction zone and/or prior to the stabilization of reaction conditions in the reactor.
Metallocene catalysts are single site catalyst which, generally, produce polymer having narrow molecular weight distribution.
While the present invention is mainly directed to xcex1-olefin homopolymerization it is obvious that copolymerization with other olefins can be obtained.
The present invention also provides for a new polymer comprising alternating blocks of syndiotactic and atactic sequences, preferably long syndiotactic and short atactic sequences, most preferably comprising a fraction of syndiotactic triads (rr) of at least 70%.